As a multicolor or single-color image forming technology, this technology has been known in that an ink sheet containing a sublimable dye is in contact with an image-receiving layer of a image-receiving element and is heated by a thermal head, etc. so that the sublimable dye is transferred imagewise to the image-receiving layer.
In such the transferred image forming technology, the image preservation quality of a post chelate sublimable dye image in which a metal chelate is formed by allowing a metal ion-containing compound (metal source) to react with a post chelate type dye (post chelate dye) employed as the sublimable dye has been epochally improved as compared to the conventional sublimable dye image.
It has been known that the image preservation quality of the post chelate sublimable dye image is improved as a chelation ratio becomes higher which is obtained by allowing a post chelate dye supplied from the ink sheet to react with the metal source in the image-receiving layer.
As a method to increase the chelation ratio, a technology has been proposed in that an image formed by transfer is treated at high temperature employing a thermostat or another heating device (Japanese Patent Publication Open to Public Inspection No. 4-89292 and Japanese Patent Application No. 5-258397). However, in this method, another device is required in addition to a thermal transfer printer and thus, a problem is caused in that the apparatus becomes large.
As a technology to solve the above-mentioned problem, a method has been proposed in that a transparent transfer foil containing a metal source is transferred onto an image formed on thermally diffusible dye-receiving layer from an ink sheet containing a thermally diffusible dye which can form a chelate complex with a metal source (Japanese Patent Publication Open to Public Inspection No. 5-42774). However, this method exhibits disadvantages such that because the transparent foil is transferred onto an image, the surface texture is altered and the metal source in the transfer foil is not efficiently supplied to dyes not involved in reaction. When the amount of the metal source in the image-receiving layer is excessively increased to increase the chelation ratio, the hue of the metal source appears on the white background of the image-receiving sheet and the hue has to be adjusted by the addition of a dye, etc. This is not preferable for the white background in terms of cost and density.